Cavitation, bubble collapse and sonoluminescence

6 problems

• Which light-emission process produces single-bubble sonoluminescence spectra?

  Partially resolved

  The flash could come from a hot gas glowing like a light-bulb filament, from free electrons braking near ions in a thin plasma (ionized gas), or from excited atoms and molecules. Which process dominates, and does it change with the gas and the liquid?

• Which collapse load, liquid jet or shock wave, causes cavitation erosion?

  Partially resolved

  A bubble collapsing next to a metal surface fires a fast liquid jet into it and also sends out shock waves, and repeated collapses pit the metal. Which of these does the damage, and how does that depend on the bubble's distance from the wall?

• Do argon rectification and gas properties explain noble-gas sonoluminescence trends?

  Partially resolved

  Air bubbles glow steadily only because nitrogen and oxygen react away and leave behind the 1 percent of argon in air, and bubbles with heavier noble gases such as xenon glow brighter. Is this chemistry, plus how well each gas conducts heat, enough to predict the brightness and stability for every noble gas?

• Peak temperature and density inside a collapsing sonoluminescent bubble

  Contested

  A single bubble trapped in a sound field collapses once per cycle and emits a flash of light lasting less than a billionth of a second. Nobody has measured directly how hot and how dense its center becomes.

• Predicting cavitation inception from turbulent vortex statistics and nuclei content

  Open

  In fast water flow, the first bubbles appear inside short-lived whirlpools where the pressure drops below the vapor pressure, but only if tiny gas seeds (nuclei) are present. Predicting the flow speed at which this starts requires knowing how deep the pressure dips get and how long seeds stay inside them.

• Why does acoustic cavitation in water fail far below quartz-inclusion strength?

  Open

  Water stretched by sound waves breaks at about minus 300 atmospheres, but tiny drops of water sealed inside quartz crystals survive about minus 1400 atmospheres. Either the sound experiments are spoiled by something not yet identified, or the textbook theory of how bubbles form is wrong for water.